Something to do with the stored energy in the flywheel ( inertia )
IE if its heavier (flywheel) then the engine will store more enegry & lose it less quicky
IE if its lighter (flywheel) then the engine will store less engergy & lose it quicker

Can't coment on the Engine baking theory though !
Can't remember that bit !
I went through this before when considering a lighter flywheel for my MK1 Golf GTI , i was directed to a web site in America who went through the pros/cons in Great detail

I have a lightened flywheel and slipper on my 853 and it definately:

1.Revs quicker
2.doesn't like to idle when cold
3.has more engine braking(i don't know why-where's Shazzam!)
4.Only get false neutrals when doddering about and not riding the thing properly.

Engine breaking is caused by the engine slowing down which as a result slows the rear wheel down, so the quicker the engine loses revs the quicker the rear wheel will slow down, lighter flywheel means the engine gains and loses revs quicker due to lower inertia, which is why you'd definitely need a slipper clutch to change down to 2nd or 1st gear in a "spirited" fashion!:cool:

Blimey!
 

maybe i need to start putting an exclamation mark next to my name:lol:

You will get more engine braking with less interia on the crank.

Think of it this way. With the engine at a high RPM, the inertia of the flywheel (or any other inertia in the drive train, such as the clutch) will want to keep the engine spinning as you close the throttle. The engine needs to absorb the energy stored in the flywheel.

So, as you reduce the inertia of the drive train, e.g. lighter flywheel, lighter clutch assembly, the engine will slow down quicker since there is less inertia trying to keep it spinning, hence more engine braking.

The downside of lighter flywheels and clutch, as pointed out already by some people, is the effect on idle. The inertia of the flywheel tends to smooth out the idle. Take away the inertia and your idle becomes lumpy. You may even need to increase the idle speed to keep it from stalling.

I run a very light flywheel and clutch and it makes for a great performing engine, but the idle is very poor. This is amplified by the particular throttle bodies, so mine may not be a good example of what you find with a street bike. I know of quite a few people that run lighter flywheels and they have almost no negative side effects. It will almost definitely make want to install a slipper clutch due to the increase in engine braking.

Pros and Cons
 

Pros and Cons

The weight of the stock flywheel is selected by Ducati test riders to provide a overall balance between performance and smooth drive train behavior. If you remove weight from the flywheel (and to a lesser degree, the clutch) the overall effect is indistinguishable from increasing your engine’s torque output.

With more torque on-hand comes the benefit of faster acceleration if you can modulate the wheel spin driving out of corners. Factory racebikes make so much power, for example, that transmitting the power to the road effectively becomes the limiting factor so heavier flywheels actually become a benefit. Fear the highside.

The ability of an engine with a lightened flywheel to spin-up more quickly is often pointed-to as a benefit when you bang a downshift and wheel chatter is reduced. In this situation, it can be said you have less engine braking. If your riding “style” makes this an issue, a slipper clutch may be a better alternative than a lightened flywheel, at least that’s what the factory racers think. A lightened flywheel is like a poor man’s slipper clutch in this situation.

The ability of an engine with a lightened flywheel to spin-down more quickly is often pointed-to as a benefit if you want the revs to die as fast as possible when you lift the throttle for a corner. In this situation, it can be said you have more engine braking. Under normal riding conditions we tend to prefer less engine braking so we tolerate a less efficient situation where the motor then has to work harder to put more momentum into the flywheel. When racing, you don’t care about storing momentum, you just want to get around the track as fast as possible so a slipper clutch is used to will reduce engine braking.

The purpose of the flywheel is to store momentum, reduce vibration and smooth out the loads transmitted to the drive train. It takes energy to first store this momentum, so if the flywheel is lighter it takes less energy and it accelerates up to speed faster. There is an opposite effect when you lift off the throttle and momentum is given up, so the revs drop slower for a heavier flywheel.

Because the engine will spin up more quickly with a lighter flywheel, you’ll have better throttle response, but modulating the throttle (especially in the rain) will be more difficult if your throttle position sensor, idle and CO are not adjusted properly. A light flywheel seem to exacerbate a poorly set-up fuel injection system. If you make 100 rear wheel horsepower it’s less of a issue. Make 130 hp and it will matter a whole lot more.

The amount of weight removed from the flywheel and inner hub is proportional to this effect, although if you remove most of the weight from the outer rim area, the effect is stronger. Different aftermarket manufacturers of flywheels offer different weights and geometry. If lightweight is good, ultra-lightweight is better doesn’t apply here. Too light a flywheel can make the bike a handfull to ride so a two pound weight reduction for the street may be a good choice.

A cold engine runs rough until there’s enough heat to vaporize the fuel, so until then, a lighter flywheel will be less effective in preventing stalls, especially pulling away from a stop uphill. You’ll need rev the engine a little higher to compensate and you may find it’s a little ornery when running at light load (3,000 to 4,000 RPM) in the lower gears in the city.

I left mine stock.

Shazaam - you da man!!!
 

Does he eat tec books or what?:ninja:

Shaz,you never fail to amaze me!